As radar systems and communication systems become more compact, there is a need for circuit elements which can be integrated in multi-level circuits. A Magic-Tee which can be integrated into planar multilevel circuits, for example microwave integrated circuits for feeding phased array antennas is particularly desirable. As is generally known, the Magic-Tee is a four port device in which (i) a signal applied at the leg of the Tee will be divided between the branch arms of the Tee, usually with equal amplitude and in-phase. The feedline is isolated; (ii) a signal applied to the feedline will be divided between the branch arms of the Tee with equal amplitude and 180.degree. out-of-phase, with the Tee leg isolated; (iii) if first and second signals are applied to the branch arms, respectively, their sum will appear at the Tee leg and their difference at the feedline; (iv) the Tee leg and the feedline are mutually isolated.
The operational band of such a Magic-Tee is the range of frequencies over which it will display these characteristics within acceptable limits. Operational bands of Magic-Tee couplers vary according to their construction. Devices implemented using waveguide technology are bulky and have a relatively narrow band width. Other Magic-Tee implementations include stripline asymmetrical couplers, which do not have good phase tracking over a wide frequency band, and microstrip-slotline configurations wherein the respective boards are not shielded from each other.
Striplines comprise a conductor between two ground planes, whereas microstrip lines comprise a conductor adjacent a single ground plane. Also, striplines and microstrip lines use different modes of propagation.
U.S. Pat. No. 5,063,365 (Capucci), which is incorporated herein by reference, discloses a Magic-Tee implemented as a stripline quadrature complex with a 90.degree. line.
U.S. Pat. No. 4,952,895 (Quan), which is incorporated herein by reference, discloses a Magic-Tee which comprises a combination of stripline and double-sided airstripline circuits formed on two dielectric boards sandwiched together. The Magic-Tee devices disclosed by Capucci and Quan have the disadvantage of non-symmetrical operation and limited performance over a wide bandwidth. The stripline structure is not readily integrable with Monolithic Microwave Integrated Circuits and relatively complex.
U.S. Pat. No. 5,075,647 (Petter) discloses a symmetrical 180.degree. microwave hybrid constructed by opening a slot line in a ground plane below a conductive strip disposed on a dielectric substrate, creating a slot-coupled conductor. In essence, Petter's device is a microstrip line--slot line--microstrip line coupler and hence uses two different modes of propagation, which is complicated. Petter's device requires proper impedance matching in two different propagating media to maximize energy coupling to the microstrip, which leads to increased size and complexity.
There remains a need, therefore, for a microwave hybrid device, especially a Magic-Tee, which is capable of wide band operation with relatively low loss, easy to construct, and suitable for use with Monolithic Microwave Integrated Circuits.